Cell division is an extremely important and complex process. Cancer cells arise when cell division is no longer regulated. The final step in the cell division cycle is cytokinesis. In most eukaryotes, cytokinesis involves the assembly and constriction of a contractile ring, an equatorial bundle of actin filaments and myosin-Il filaments. Our long range goal is to understand the role of myosin-Il in cytokinesis. Fission yeast provides an excellent genetically tractable model system to study cytokinesis. I propose to use a combination of genetics, biochemistry and live-cell microscopy to investigate myosin-II in fission yeast. Initially, genes required for myosin-Il function will be identified by cloning suppressors of myosin-Il temperature-sensitive mutants. Subsequently, the assembly and contraction of the contractile ring will be analyzed in live cells by visualizing GFP-tagged myosin-Il in fission yeast cells that contain mutations in the cytokinesis components that were identified. Finally, the specific mechanism of myosin-Il function will be investigated by purifying myosin-Il and suppressor proteins and reconstituting their interaction in vitro. This combination of experimental strategies is essential for investigating cytokinesis at both the whole-cell and individual molecule level.